Added functionality for dealing with cancellations/deposit.
This involved moving some data from 'htmlchunks' to the template for
simplicity, in order to display the deposit value, and creating one new
'htmlchunk'.

/booking/start/ now redirects to basket if there is anything in it.
Involved some refactoring of test code, and this in turn involved changing
the webfaction session code to not throw errors when in test mode, otherwise
tests were failing due to tracebacks in the outbox.